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Abstract Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area.

Abstract We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest $${\mathrm{M}}_{\mathrm{w}}$$ M w 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount. Graphical Abstract

Abstract The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on March 11 and generated strong shaking and huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous large aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom. To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea bottom-water environment off Sanriku, we conducted shipboard surveys up to ~ 2000 dbar during 2011–2018 and long-term monitoring of the seafloor on the continental slope using a deep-sea station (~ 1000 dbar) off Otsuchi Bay during 2012–2018. The high turbidity (maximum ~ 6%) was observed for the bottom water deeper than 500 dbar on the continental slope of the entire area off Sanriku during 2012–2018. This high turbidity was caused by sporadic sediment resuspension induced by frequent large aftershocks. Furthermore, dissolved oxygen concentrations in the bottom layer from 1000 to 1500 dbar dropped significantly by about 10% after the earthquake, while nutrients and dissolved inorganic carbon showed no significant changes but exhibited wide variations. The high turbidity was associated with the increase in the concentrations of phosphate, dissolved inorganic carbon, and methane, as well as the decrease in those of dissolved oxygen and nitrate. This suggests that remineralization of suspended organic matter resulting from the respiration and denitrification of microbial communities after the earthquake caused the chemical properties of the deep-sea bottom-water. The deep-sea bottom-water environment change was maintained by sporadic sediment resuspension due to continued large aftershocks and was likely caused by variations in dissolved inorganic carbon and phosphate. There are two peaks in the concentration and carbon isotope ratio of methane on the deeper slope from 1000 to 2000 dbar near the hypocenter, which were advected along isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). The source of the shallower peak of chemical input is considered to be the sediment resuspension from the shallow sediment on the continental slope induced by the mainshock and large aftershocks. Graphical abstract

Copyright of Journal of the Sedimentological Society of Japan / Taisekigaku Kenkyu is the property of Sedimentological Society of Japan and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area. [ABSTRACT FROM AUTHOR]

We conducted centroid moment tensor (CMT) inversion for small-to-moderate aftershocks in the off Ibaraki region of the 2011 off the Pacific coast of Tohoku earthquake. In this study, we used high-frequency (0.4–1.0 Hz) seismograms from a dense array of short-period ocean bottom seismometers (OBSs) and a reliable three-dimensional (3-D) seismic velocity model. Higher frequency analysis and dense OBS arrays offer CMT solutions with high spatial resolutions. Since our OBS array observed aftershocks occurring immediately following the 2011 off the Pacific coast of Tohoku earthquake, we determined 536 CMT solutions for small-to-moderate aftershocks in the off Ibaraki region (JMA-scale magnitudes of 2.5–4.0). According to our CMT solutions, characteristics of the aftershock activities in the off Ibaraki region are classified into 5 groups: (1) thrust earthquakes, which are considered as interplate earthquakes, separated by the large slip area of the 2011 Ibaraki-oki earthquake (the largest M w 7.6 aftershock of the 2011 off the Pacific coast of Tohoku earthquake) and the tectonic tremors; (2) intraslab strike-slip earthquakes located at the north of the fault area of the 2011 Ibaraki-oki earthquake; (3) intraslab normal-fault earthquakes, which suggest a tensional stress field within the subducting Pacific Plate due to the plate bending by the overriding Philippine Sea Plate; (4) various earthquake focal mechanisms above a subducting seamount, which suggest the 3-D complex fractures; and (5) normal-fault earthquakes shallower than the interplate earthquakes, which were possibly caused by the heterogeneity of the subducting seamount. [ABSTRACT FROM AUTHOR]

The 2011 off the Pacific coast of Tohoku Earthquake occurred off southern Sanriku, Japan, on March 11 and generated strong shaking and huge tsunami along the entire eastern coast of Tohoku. The mainshock and numerous large aftershocks caused mass-wasting sedimentation events on the continental shelf, slope, and trench bottom. To investigate the impacts of the 2011 Tohoku Earthquake on the deep-sea bottom-water environment off Sanriku, we conducted shipboard surveys up to ~ 2000 dbar during 2011–2018 and long-term monitoring of the seafloor on the continental slope using a deep-sea station (~ 1000 dbar) off Otsuchi Bay during 2012–2018. The high turbidity (maximum ~ 6%) was observed for the bottom water deeper than 500 dbar on the continental slope of the entire area off Sanriku during 2012–2018. This high turbidity was caused by sporadic sediment resuspension induced by frequent large aftershocks. Furthermore, dissolved oxygen concentrations in the bottom layer from 1000 to 1500 dbar dropped significantly by about 10% after the earthquake, while nutrients and dissolved inorganic carbon showed no significant changes but exhibited wide variations. The high turbidity was associated with the increase in the concentrations of phosphate, dissolved inorganic carbon, and methane, as well as the decrease in those of dissolved oxygen and nitrate. This suggests that remineralization of suspended organic matter resulting from the respiration and denitrification of microbial communities after the earthquake caused the chemical properties of the deep-sea bottom-water. The deep-sea bottom-water environment change was maintained by sporadic sediment resuspension due to continued large aftershocks and was likely caused by variations in dissolved inorganic carbon and phosphate. There are two peaks in the concentration and carbon isotope ratio of methane on the deeper slope from 1000 to 2000 dbar near the hypocenter, which were advected along isopycnal surfaces of 27.38σθ (1000 dbar) and 27.56σθ (1500 dbar). The source of the shallower peak of chemical input is considered to be the sediment resuspension from the shallow sediment on the continental slope induced by the mainshock and large aftershocks. [ABSTRACT FROM AUTHOR]

The hinge type of precast concrete arch culvert was introduced to Japan from France in the 1990s in consideration of the saving of labor, shortening of the construction period, and high quality control of the concrete members. However, due to the 2011 off the Pacific Coast of Tohoku Earthquake (March 11, 2011), the three-hinged precast arch culverts that had been constructed in Japan at the beginning of the period when precast arch culverts were firstly introduced, suffered damage, which spoiled their serviceability. According to the extent of the damage and the type of culverts that were damaged, the longitudinal structural connectivity of the culverts was assumed to be one of the possible reasons for the reported damage mechanism. Therefore, the objective of this paper was to clarify how strongly the longitudinal structural connectivity influenced the longitudinal seismic behavior of the three-hinged arch culverts. To achieve this objective, an elasto-plastic finite element analysis was conducted with an analytical model that could capture the characteristics of the damaged culverts. Simultaneously, a penalty method with the bi-linear spring model was applied as a solution to the contact-impact problems of the precast segmental arch members. As a result, it was found that the weaker longitudinal structural connectivity in the damaged culverts allowed the torsional displacements of the arch members to induce critical damage to the arch members, namely, edge defects in the arch crown and concrete foundation. The numerical results proved the unignorable influence of the longitudinal structural connection on the possible damage to three-hinged arch culverts.

The hinge type of precast concrete arch culvert was introduced to Japan from France in the 1990s in consideration of the saving of labor, shortening of the construction period, and high quality control of the concrete members. However, due to the 2011 off the Pacific Coast of Tohoku Earthquake (March 11, 2011), the three-hinged precast arch culverts that had been constructed in Japan at the beginning of the period when precast arch culverts were firstly introduced, suffered damage, which spoiled their serviceability. According to the extent of the damage and the type of culverts that were damaged, the longitudinal structural connectivity of the culverts was assumed to be one of the possible reasons for the reported damage mechanism. Therefore, the objective of this paper was to clarify how strongly the longitudinal structural connectivity influenced the longitudinal seismic behavior of the three-hinged arch culverts. To achieve this objective, an elasto-plastic finite element analysis was conducted with an analytical model that could capture the characteristics of the damaged culverts. Simultaneously, a penalty method with the bi-linear spring model was applied as a solution to the contact-impact problems of the precast segmental arch members. As a result, it was found that the weaker longitudinal structural connectivity in the damaged culverts allowed the torsional displacements of the arch members to induce critical damage to the arch members, namely, edge defects in the arch crown and concrete foundation. The numerical results proved the unignorable influence of the longitudinal structural connection on the possible damage to three-hinged arch culverts. [ABSTRACT FROM AUTHOR]

More available warning time and a quicker rapid intensity reporting after earthquake are of great significance to the earthquake early warning system and the after earthquake rescue. The improvement of the calculation speed of the instrument intensity and the accuracy of the calculation results are very important for the whole early warning process. The development of intensity in Japan has gone through three stages：instrument intensity of JMA in 1996，real-time instrument intensity in 2008 and real-time instrument intensity in 2013. This paper explains these three and gets the contract picture by using the data in the 2011 off the pacific coast of Tohoku earthquake. The instrument intensity using 2013 method is quicker than that using 1996 method，and the instrument intensity using 2013 method is closer to that using 1996 method，compared with that using 2008 method.

Abstract We investigated temporal changes in the waveforms of S-coda from triggered earthquakes around the Moriyoshi-zan volcano in northeastern Japan. Seismicity in the area has drastically increased after the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. We analyzed distinct scattered wave packets (DSW) that are S-to-S scattered waves from the mid-crust and appeared predominantly at the high frequency range. We first investigated the variation of DSW for event groups with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Despite the above restriction, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal trends were much more complicated in many cases. We also found that the shape of DSW changed in a very short period of time, for example, within ~ 12 h. Next, we estimated the location of the origin of the DSW (DSW origin) by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster of triggered events. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a tomographic study. This spatial distribution implies that the DSW origin was composed of geofluid-accumulated midway in the upward fluid movement from the low-velocity zone to the earthquake cluster. Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the temporal changes in DSW waveforms possibly reflect the temporal changes in scattering properties in and/or near the origin. The quick change in DSW waveforms implies that fast movement of geofluid can occur at the depth of the mid-crust.

In this study, the on-site P-wave earthquake early warning (EEW) based on the site-specific spectral ratio of S-wave to P-wave to efficiently incorporate the site characteristics, which can potentially issue the earthquake warning by the time of Ts-p, was developed. The spectral ratio of S-wave to P-wave that are related to the source effects, the path effects, and the site effects are significantly affected by the site effects contrast to the source effects and the path effects in practical. At first, the on-site P-wave EEW method which multiplies a site-specific spectral ratio of S-wave to P-wave prepared in advance by P-wave observed in the real-time at seismic stations is applied to seismic data for moderate-sized earthquakes with a magnitude (Mj) of 5.0–6.0, occurred in the eastern Japan, observed at both the sedimentary basin site and the rock site. As a result, this method predicted well the observed S-wave in the single indicator of SI within the logarithmic standard deviation of 0.25 as well as in the frequency of more than 0.5 Hz. It is, also, confirmed that the site-specific spectral ratio of S-wave to P-wave at a seismic station was stably retrieved from 20 data samples at least. To investigate the applicability of this method to earthquake ground motions induced by a large-scaled earthquake, finally, this method is applied to seismic data during the 2011 off the Pacific coast of Tohoku earthquake, Japan (Mw 9.0). The prediction of S-wave using a time-window of 10 s after P-wave arrived, could not reproduce the observation with the underestimation; however, the prediction of S-wave using a time-window of more than 20 s containing P-wave propagated from an area generating strong motions in the fault, could reproduce the observation. Even in the case of the large-scaled earthquake, the on-site P-wave EEW method based on the site-specific spectral ratio of S-wave to P-wave at a seismic station availably works by using the gradually increasing time-windows after P-wave arrived in the single indicator of SI as well as in the frequency content, avoiding the mixture of S-wave into a part of P-wave.

In this study, the distribution and characteristics of the tsunami deposits by the 2011 off the Pacific coast of Tohoku and A.D. 869Jogan earthquakes were identified in Hirota Bay, Sanriku Rias through high-resolution seismic profiles and core samples. The seabed sediments of Hirota Bay are composed of sand layer and silt layer, and separated by a well-defined boundary. The tsunami deposit by the 2011 Tohoku-oki earthquake on the Hirota Bay seafloor exhibit the characteristics of: (1) the bed separated from lower layer by a clear distinct boundary, (2) coarser grain size than ordinary bay-floor sediment, (3) composed by more than two sand layers, (4) parallel lamination, and (5) broad distribution within Hirota Bay. The volume of the 2011 tsunami deposit is 2.6x106 m³, about four times larger than the coeval onshore tsunami deposit in the Rikuzentakata Plain. The total volume of these onshore and offshore tsunami deposits is greater than the total volume of coastal erosion ranging from the beach ridge to shallow-marine. Hence, the sediment sources of the 2011 Tohoku-oki tsunami deposit may also include the materials from the coasts (east and west sides of bay) and deeper seafloor of Hirota Bay, which are not included in the calculation for the volume of erosion. Another event layer occurs beneath the 2011 Tohoku-oki tsunami deposit in Hirota Bay. This layer is interpreted as A.D. 869Jogan tsunami deposit based on their distribution, sediment facies and radiocarbon-ages. [ABSTRACT FROM AUTHOR]

On March 11, 2011, a large earthquake of Mw 9.0 shook north-eastern Japan and caused severe liquefaction-induced damage over a wide area of reclaimed lands along the coast of Tokyo Bay. Although regional mapping of the liquefaction hazard had been performed in many automounts bodies in Japan, it seems that the maps were not effectively used on all fronts of disaster-prevention management, because the maps only provide liquefaction susceptibilities and little quantitative information on how seriously the ground might deform in a scenario earthquake, which is absolutely necessary information for discussing what-if scenarios. Konagai et al. (2013) conducted air-borne LiDAR surveys to obtain liquefaction-induced ground deformations over the north-eastern stretch of the Tokyo Bay shore area, including Urayasu City, where approximately 85% of the city area was heavily liquefied. Meanwhile, the authors have developed a geotechnical database for liquefaction risk assessments, compiling all the available borehole logs and soil testing data provided by Urayasu City (2012). Given the potential risk of re-liquefaction in a future scenario earthquake, it is an overriding priority to develop a knowledge-sharing liquefaction hazard map reflecting precise records from the past, such as liquefaction-induced ground subsidence and liquefaction-related damage. This paper attempts to assess the liquefaction-induced damage risk on road network, examining the relationship between the liquefaction potential index and the actual ground subsidence. For this purpose, firstly, the spatial distribution of the liquefaction potential (P L) was estimated over Urayasu City based on the above-mentioned geotechnical database developed by the authors. Secondly, the spatial distribution of the P L values and the actual liquefaction-induced road subsidence confirmed through air-born LiDAR surveys were compared to develop an empirical rule for estimating the potential road subsidence in a scenario earthquake. This empirical rule was found to describe the actual damage to roads and manholes in a satisfactory manner. Therefore, it is expected that a risk map, developed on the basis of this empirical rule, will not only help to assess liquefaction-induced damage, but also to design countermeasures against the what-if scenarios of liquefaction. [ABSTRACT FROM AUTHOR]

Abstract A method for an evaluation of the building structure response reduction by soil‐structure interaction (SSI) based on the seismographs of the soil‐structure system is presented. In the proposed method, a simplified SSI model is employed, and the parameters of model are determined by system identification. For system identification, both the inertial interaction (II) and kinematic interaction (KI) are considered. The method that indicates a reduction in the structure response by II and KI separately is also presented. The proposed procedures are applied to the observed records of the 2011 off the Pacific coast of Tohoku Earthquake. The findings demonstrate that the reduction effect of II is reduced when the response of building structure enters the inelastic range, and vice versa. In the superstructures with pile foundations, the reduction effects of KI are relatively smaller than those of II.

Abstract The authors performed static analyses using a foundation structure model to simulate the process leading to distinctive damage to pile foundations during the 2011 off the Pacific coast of Tohoku earthquake. Notable aspects of the analytical method used for this simulation include consideration of the nonlinear load‐deformation characteristics of pile elements, the dependence of the rotational stiffness at the pile head on the axial load, and the nonlinear behavior of soil spring including the pile group effect. The results of the simulations of pile failure for each loading direction provide a useful explanation of the observed damage to pile foundations. The proposed analysis method can be considered a practical approach to the seismic design of foundations subject to severe earthquakes.

We investigated observational data from 20 geomagnetic observatories at longitudes of 130°E to 150°E between 2009 and 2012 in order to detect possible anomalous changes in the geomagnetic solar quiet daily (Sq) variation related to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) that occurred on March 11, 2011, in Japan. We selected the five quietest days in every month and analyzed three-component magnetic data with 1-min resolution using the natural orthogonal component method to identify Sq variations from observations, and performed spherical harmonic analysis (SHA) to separate the internal and the external equivalent Sq current systems of Je and Ji. We found that Je and Ji were distorted near the epicenter on two geomagnetic quiet days, January 23 and 27, 2011. This was about 2 months before the earthquake. We then employed backward computation on the external and internal parts of the SHA model geomagnetic fields, Be and Bi, to determine the possible anomalous magnetic field variations at each station. The results showed significant differences in daily variation for all components on January 23 and 27, 2011. On the other days, we could not recognize any apparent anomalous behaviors apart from known secular, seasonal, and solar activity dependence. A possible connection between this anomaly and the very strong Tohoku earthquake, which followed only 1.5 months after the geomagnetic anomaly, is of extreme interest for research on earthquake precursors. [ABSTRACT FROM AUTHOR]

Copyright of Geotechnical Engineering (00465828) is the property of Southeast Asia Geotechnical Society (SEAGS) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Based on previous studies on the seismic monitoring of isolated (IS) and non-isolated (NI) buildings supported by piled rafts with a DMW (deep mixing wall) grid, under similar soft-soil conditions during the 2011 Tohoku earthquake, the effects of kinematic and inertial forces on the piled rafts, for which the natural periods of the superstructures were significantly different, were investigated. To evaluate the degree of contribution of the kinematic and inertial forces on the maximum bending moments and incremental axial forces near the pile heads, the coefficients related to the kinematic and inertial forces were presented. The coefficients of inertial force were separated into those of the superstructure and raft inertias to consider the embedment effects. Furthermore, the mechanisms of the inelastic behavior of the settlement and the load transfer in the piled raft systems, observed during the principal motion, were discussed. Consequently, it was found that the ratio of the natural period of the soil-pile-structure system to the predominant period of the ground, T s / T g , plays an important role in the contribution of the kinematic and inertial forces to the maximum bending moments near the pile head. Moreover, the T s / T g values were seen to be closely related to the differences in the inelastic settlement and load transfer mechanisms of the piled raft systems. [ABSTRACT FROM AUTHOR]

A method for an evaluation of the building structure response reduction by soil-structure interaction (SSI) based on the seismographs of the soil-structure system is presented. In the proposed method, a simplified SSI model is employed, and the parameters of model are determined by system identification. For system identification, both the inertial interaction (II) and kinematic interaction (KI) are considered. The method that indicates a reduction in the structure response by II and KI separately is also presented. The proposed procedures are applied to the observed records of the 2011 off the Pacific coast of Tohoku Earthquake. The findings demonstrate that the reduction effect of II is reduced when the response of building structure enters the inelastic range, and vice versa. In the superstructures with pile foundations, the reduction effects of KI are relatively smaller than those of II. [ABSTRACT FROM AUTHOR]

The authors performed static analyses using a foundation structure model to simulate the process leading to distinctive damage to pile foundations during the 2011 off the Pacific coast of Tohoku earthquake. Notable aspects of the analytical method used for this simulation include consideration of the nonlinear load-deformation characteristics of pile elements, the dependence of the rotational stiffness at the pile head on the axial load, and the nonlinear behavior of soil spring including the pile group effect. The results of the simulations of pile failure for each loading direction provide a useful explanation of the observed damage to pile foundations. The proposed analysis method can be considered a practical approach to the seismic design of foundations subject to severe earthquakes. [ABSTRACT FROM AUTHOR]

The 2011 Off the Pacific Coast of Tohoku earthquake (Tohoku earthquake, M 9.0) occurred on the Japan Trench and caused a devastating tsunami. Studies of this earthquake have revealed complex features of its rupture process. In particular, the shallow parts of the fault (near the trench) hosted large slip and long period seismic wave radiation, whereas the deep parts of the rupture (near the coast) hosted smaller slip and strong radiation of short period seismic waves. Understanding such depth-dependent feature of the rupture process of the Tohoku earthquake is necessary as it may occur during future mega-thrust earthquakes in this and other regions. In this study, we investigate the 'characterized source model' of the Tohoku earthquake through dynamic rupture simulations. This source model divides the fault plane into several parts characterized by different size and frictional strength (main asperity, background area, etc.) and is widely used in Japan for the prediction of strong ground motion and tsunami through kinematic rupture simulations. Our characterized source model of the Tohoku earthquake comprises a large shallow asperity with moderate frictional strength, small deep asperities with high frictional strength, a background area with low frictional strength, and an area with dynamic weakening close to the trench (low dynamic friction coefficient as arising from, e.g., thermal pressurization). The results of our dynamic rupture simulation reproduce the main depth-dependent feature of the rupture process of the Tohoku earthquake. We also find that the width of the area close to the trench (equal to the distance from the trench to the shallow asperity, interpreted as the size of the accretionary prism) and the presence of dynamic weakening in this area have a significant influence on the final slip distribution. These results are useful to construct characterized source models for other subduction zones with different scale of the accretionary prism, such as the Chile subduction zone and the Nankai Trough. Dynamic rupture simulations based on the characterized source model might provide useful insights for hazard assessment associated with future mega-thrust earthquakes. [ABSTRACT FROM AUTHOR]

Ofunato Bay was a semi-closed area because of the breakwater effect at the entrance; however, the breakwater was destroyed by a massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake. Consequently, the physical environment of Ofunato Bay has been changed significantly, i.e., the modification of the stratified structure of seawater inside the bay and the intermittent intrusion of seawater outside the bay. These alterations of physical environment are considered to have an influence on the chemical and biological environment in Ofunato Bay. To elucidate the influence of the tsunami on the aquatic environment, we measured dissolved nutrients, chlorophyll a and dissolved oxygen concentrations, and heterotrophic bacteria abundance inside and outside of Ofunato Bay from 2012 to 2014, and compared these data with those obtained before the earthquake. As compared with before the earthquake, significant changes after the earthquake were (1) decrease of ammonium and phosphate concentrations, (2) increase of chlorophyll a concentration, (3) increase of dissolved oxygen concentration in the bottom, and (4) decrease of heterotrophic bacteria abundance. The collapse of the breakwater and consequential enhanced water exchange were considered to have brought the decrease of nutrient concentration inside the bay. Furthermore, washout of shellfish mariculture rafts by the tsunami decreased the shellfish biodeposits along with the elution of nutrients by heterotrophic bacteria. Decrease of cultivated shellfish further caused a decline in feeding pressure on phytoplankton and, subsequently, increased the phytoplankton biomass that contributed to the decrease of nutrients inside the bay. [ABSTRACT FROM AUTHOR]

This study investigates the effects of quality of in-situ sandy samples, which were taken by a conventional tube sampling (TS) method, on liquefaction assessment. The sandy samples were retrieved from a loose fill layer and medium-dense Holocene deposit in young reclaimed land in Urayasu City where significant liquefaction occurred during the 2011 off the Pacific Coast of Tohoku Earthquake. First, a series of small strain measurements and undrained cyclic triaxial tests was conducted on the TS and reconstituted samples. Second, in order to investigate an applicability of the tested samples for the liquefaction assessment, an effective stress analysis was performed by use of soil parameters obtained from the examined samples. Input earthquake motions of the analysis are the 2011 off the Pacific Coast of Tohoku Earthquake and the 1987 Chiba Toho-oki Earthquake, the first and the second largest earthquakes the investigation site has experienced. The result of a sample quality assessment with the small strain shear moduli indicates that the TS sample retrieved from the Holocene deposit remains of high quality. In addition, for the reconstituted sample of the fill layer, since the small strain shear moduli of the sample are found to be nearly equal to those obtained at the field, it can be considered that the in-situ soil structure condition is reproduced in the reconstituted sample. According to an official report, the liquefaction resistance of the TS sample of the loose fill layer is R L20 =0.38, which is much larger than that of medium-dense Holocene deposit ( R L20 =0.29) measured in this study. This is because the fill layer is so loose that the TS sample could have been strengthened due to disturbance during the sampling. On the other hand, the test result for the reconstituted sample of fill layer shows an appropriate level of liquefaction resistance ( R L20 =0.14). The results of the effective stress analysis using the soil parameters obtained from the TS sample of Holocene deposit and the reconstituted sample of fill layer show the most probable behaviors of the ground at the investigation site for both the 2011 off the Pacific Coast of Tohoku Earthquake and the 1987 Chiba Toho-oki Earthquake, which are consistent with the evidence reported after the earthquakes. The results indicate that the sample quality assessment by using a small strain shear moduli is applicable for the rational liquefaction assessment, and it would be possible to employ the reconstituted sample, which the small strain shear moduli are the same with the in-situ value, in substitution for a high quality sample. [ABSTRACT FROM AUTHOR]

This paper offers a case history of a friction piled raft foundation supporting a seven-story building on soft ground in Tokyo. Since the building is located on loose sandy soil, underlain by very soft clayey soil, grid-form cement deep mixing walls (DMWs) were employed as a countermeasure against soil liquefaction. To confirm the validity of the foundation design, field monitoring of the foundation settlement and the load sharing between the piles and the raft was performed over eleven years from the beginning of the construction. At the time of the 2011 off the Pacific coast of Tohoku Earthquake, the seismic response of the soil–foundation–structure system was successfully recorded, namely, the peak horizontal acceleration of 1.21 m/s 2 was recorded on the first floor of the building. Before the earthquake, the measured settlements of the first floor were 15–29 mm; after the event, the maximum incremental settlement was 6 mm. The ratio of the load carried by the piles to the net load in the tributary area was estimated to be 0.72 before the event and increased to 0.81 at maximum about three years after the event. Based on the seismic observations, it is confirmed that the residual settlements of the piled raft were caused mainly by the penetration of the piles subjected to the moment load from the superstructure. On the other hand, it is suggested that the bending moment near the pile head was caused mainly by the horizontal ground displacement near the surface, rather than the inertial force of the superstructure. It was also found that the bending moment near the pile head was significantly small, as is expected in the design of piled raft systems. This may arise because the raft had a relatively small number of piles and was in contact with both the grid-form DMWs and the soil enclosed by them, where most of the horizontal load would be carried by the raft. Consequently, it was found that the friction piled raft foundation combined with the grid-form DMWs showed a good performance in a ground consisting of liquefiable sand and soft cohesive soil under both seismic and ordinary conditions. [ABSTRACT FROM AUTHOR]

On 11 March 2011, the Sanriku coast of Japan was struck by a massive tsunami associated with the 2011 off the Pacific coast of Tohoku Earthquake. The tsunami caused large disturbances in bottom sediments and severe destruction of human-made structures, natural landscapes and infrastructures such as sewage treatment systems and aquaculture facilities. A serious and continuing concern is a fundamental alteration of the nutrient regime in the impacted regions, which may exert a profound influence on the processes of ecosystem recovery and fishery reconstruction. We investigated the nutrient status of the water column of Otsuchi Bay, a small embayment that was heavily damaged by the tsunami. Our data collected between May 2011 and July 2014 revealed some prominent features in nutrient parameters that were potentially linked to effects of the tsunami. These features include (1) high turbidity and high concentrations of nitrite and silicic acid until September 2011, (2) the spread of seawater characterized by high phosphate concentration and low total inorganic nitrogen (ammonium + nitrate + nitrite) to phosphate (TIN/P) ratio during the mixing period between November 2011 and January 2012, and (3) an increase in the TIN/P ratio during the mixing period of subsequent years to become higher than it was before the earthquake. These data not only provide evidence of the alteration and subsequent recovery of the nutrient status of Otsuchi Bay after the earthquake but also suggest a possible tsunami-induced alteration in nutrient stoichiometry which appeared to last until at least the beginning of 2014. [ABSTRACT FROM AUTHOR]

The objective of this paper is to clarify the factors that resulted in many landslides during the 2011 off the Pacific Coast of Tohoku Earthquake. The geology of the Tohoku district, the seismic motion of the main shock and the precipitation around the time of the earthquake are described. As one of the characteristic phenomena observed around Shirakawa city in Fukushima prefecture and the mid-eastern part of Tochigi prefecture, many of the slopes were characterized by a high distribution of sensitive volcanic cohesive soils. The estimated PGA and PGV around the areas were therefore larger, which might have promoted the softening of the sensitive soils. Although there had been little precipitation before the main shock, the landslides occurred mostly in the slopes containing the volcanic cohesive soils with higher water content. The influences of the ground water as well as other factors on the seismic slope stability are discussed in each of the following case studies. In order to clarify the mechanism of each landslide, the study focused on three typical landslides which occurred in the affected areas, i.e., the Hanokidaira, Okanouchi and Kami-kashiwazaki landslides. Detailed surveying with a laser profiler (LP) was carried out at the Hanokidaira and Okanouchi landslides to enable the topography of each slope before and after the main shock to be accurately grasped. Analytical examinations for the sliding mechanisms in each slope from an engineering point of view are made based on the numerical analyses with the actual soil properties observed in the laboratory tests. According to the results obtained from the cyclic direct and triaxial tests using the undisturbed specimens of the clayey loams sampled at the slip surfaces in each landslide, the shear strength can be reduced under cyclic loading and may eventually make the materials behave as those of high-mobility. Strength reduction is believed to be caused by the destruction of the clay particle structures originated from the sedimentation. Since the duration of the main shock exceeded three minutes, the slopes were subjected to the strong motion for a very long time. The sensitive cohesive soils resulted in a state with high mobility, which might have been the reason for the long distances traveled by the debris. [ABSTRACT FROM AUTHOR]

The present text is a summary of liquefaction events that occurred in the Kanto Region during the 2011 off the Pacific coast of Tohoku Earthquake, excluding those events in the Tokyo Bay area that are discussed elsewhere. Most liquefaction occurred in the abandoned river channels and former lakes that had been recently filled with liquefiable soils. Noteworthy was the damage to private housing lands and river dikes that had not been well treated against liquefaction risk. Many sites experienced repeated liquefaction after former earthquakes in 1923 or 1987, if they existed in those times. In contrast to the recent liquefaction-prone soils, more-aged sands scarcely liquefied because of what is called the “ageing” effect. The present study analyzed cases in Tokyo area to quantitatively assess the temporal development of ageing so that liquefaction risk in more aged soils might be reasonably assessed. In the appendix, remarks are made of the reliability of air-photo survey in quick detection of liquefaction sites. [ABSTRACT FROM AUTHOR]

On March 11, 2011, the largest earthquake ever recorded in Japan struck off the coast of the Tohoku region of the country, and was subsequently named the 2011 off the Pacific coast of Tohoku Earthquake by the Japan Metrological Agency. The M w 9.0 earthquake generated strong motions that affected the island of Honshu from Tokyo Bay to the northern extent of the island, and induced a series of tsunamis that devastated coastal communities throughout the region. Significant aftershocks ( M w >7) were experienced that further contributed to damage in the coastal Tohoku region during emergency response and recovery efforts. This paper summarizes the findings of the Port and Airport Research Institute (PARI) research team as it investigated coastal structures along approximately 600 km of coastline as well as the team׳s follow-up experiments and analyses. Two characteristics of the ground motion observed were the long duration and the high-frequency component. For this reason, the degree of damage caused to coastal facilities by the ground motion was relatively small. The wide-ranging investigation by the PARI team facilitated the interpretation of damage patterns across the entire region affected by the earthquake, with a primary goal of distinguishing damage to coastal structures resulting from strong ground shaking and secondary effects (such as liquefaction, ground failures, and settlement) from that caused by the subsequent and significant tsunami inundation. [ABSTRACT FROM AUTHOR]

Since many infrastructures, such as sea walls, sand beaches, forests, etc., were severely damaged or destroyed by the tsunami that occurred due to the 2011 Off the Pacific Coast of Tohoku Earthquake, it is said that the resistant functions of the above structures against tsunami attacks did not perform well. However, some structures are known to have resisted the tsunami, based on field surveys conducted after the earthquake by the authors and others; and thus, the resistant functions of those infrastructures against tsunami should be estimated more properly. This paper focuses on earth structures, including river levees and road embankments, both damaged and undamaged, at 13 sites in Miyagi Prefecture, Chiba Prefecture, and Ibaraki Prefecture. They have been investigated through field surveys and other related data, such as satellite photographs taken before and/or after the tsunami. Furthermore, 10 dug pools, eroded by the flood stream on the back side of sea walls and banks during the tsunami, are also investigated to clarify their effects against tsunami attacks for use as future hardware countermeasures. Based on the above field investigations, several important lessons on hardware countermeasures against tsunami, using earth structures, are discussed. And, performance-based design concepts for reconstruction after this earthquake and for the reduction of future tsunami damage are discussed and proposed. [ABSTRACT FROM AUTHOR]

Abstract: This paper gives an overview of the crustal deformation caused by “the 2011 off the Pacific Coast of Tohoku Earthquake” (hereinafter: “the Tohoku Earthquake”) as detected by the GPS Earth Observation Network (GEONET), a GPS continuous observation system operated by the Geospatial Information Authority of Japan (GSI), and of ground deformation resulting from liquefaction triggered by the earthquake motion as determined by a leveling survey. A very large part of the Japanese archipelago was affected by the crustal deformation caused by the mainshock. The subsidence in the Pacific coastal area of the Tohoku region is especially remarkable. Based on a resurvey after the earthquake, the GSI revised the coordinates and heights of survey marks, including the Origin of the Japanese Horizontal Control Network and the Origin of the Japanese Vertical Control Network. We estimated the geometry of the seismogenic fault of the Tohoku Earthquake, as well as a slip model for the boundary between the Pacific Plate and the North American Plate, based on crustal deformation data. Postseismic crustal deformation was also observed by GEONET. Even though the area that coseismicaly subsided is undergoing partial uplift from postseismic deformation, such uplift is not rapid enough to make up for the coseismic subsidence within the next several years. The earthquake caused liquefaction in large parts of the Kanto Region, especially on the Tokyo Bay side and at the lower reaches of the Tone River. Liquefaction was concentrated in areas of land reclamation and former river channels, where it is easy to recognize using time-series geospatial information such as olden topographic maps and olden aerial photographs. To determine the amount of land subsidence (cm) due to liquefaction in Urayasu City, the GSI carried out a leveling survey and compared the differences between pre- and post-earthquake LIDAR survey data with the relative subsidence obtained by leveling. [Copyright &y& Elsevier]

Abstract: The static and seismic behavior of a piled raft foundation, supporting a 12-story base-isolated building in Tokyo, is investigated by monitoring the soil–foundation–structure system. Since the building is located on loose silty sand, underlain by soft cohesive soil, a piled raft with grid-form deep cement mixing walls was employed to cope with the liquefiable sand as well as to improve the bearing capacity of the raft foundation. To confirm the validity of the foundation design, field measurements were carried out on the ground settlements, the pile loads, the contact pressure and the pore-water pressure beneath the raft from the beginning of the construction to 43 months after the end of the construction. On March 11, 2011, 30 months after the end of the construction, the 2011 off the Pacific coast of Tohoku Earthquake struck the building site. The seismic response of the ground and the foundation–structure system was successfully recorded during the earthquake, and a peak horizontal ground acceleration of 1.75m/s2 was observed at the site of the building. Based on static and dynamic measurement results, it was found that there was little change in the foundation settlement and the load sharing between the raft and the piles before and after the earthquake. It was also found that the horizontal accelerations of the superstructure were reduced to approximately 30% of those of the ground near the ground surface by the input losses due to the kinematic soil–foundation interaction in addition to the base isolation system. Consequently, the piled raft with grid-form deep cement mixing walls was found to be quite stable in the soft ground during and after the earthquake. [Copyright &y& Elsevier]

Abstract: Earthquake reconnaissance investigations were carried out at regions located along the lower stream of Tonegawa River where widely spread soil liquefaction and associated phenomena were observed following the main shock and aftershocks of the 2011 off the Pacific coast of Tohoku Earthquake. A multiple series of Swedish Weight Sounding tests were carried out at Sawara of Katori City, Hinode of Itako City, and Wanigawa and Fukashiba of Kamisu City. The soil profiles responsible for the damage inflicted by soil liquefaction were demonstrated along with the help of SPT data. At some locations where the soil samples of erupted sand boils were retrieved, the profiles of relative density and factor of safety against liquefaction with depth are estimated and discussed in detail. The use of Swedish weight sounding tests along with a help of SPT soil profiling proved an effective method for detecting liquefiable soil layers responsible for widespread soil liquefaction induced damage. [Copyright &y& Elsevier]

Abstract: This paper describes the building damage incurred during the 2011 off the Pacific coast of Tohoku Earthquake in relation to ground motion characteristics. First, with regard to the long-duration ground motion, the damage of a base-isolation building is described. The damage to the 8–9-story buildings in the Aobayama campus of Tohoku University, in a hilly zone, is also described in relation to site-specific ground motion amplification. As specific building damage, pile foundation damage is described. The ceiling board drop damage is also presented as an example of non-structural element damage. Finally, the lessons learnt from the earthquake in terms of building damage are summarized. [Copyright &y& Elsevier]

Abstract: Immediately after the 2011 off the Pacific coast of Tohoku Earthquake, special safety inspections were carried out at over 300 dams in the affected area. Damage was identified at more than 10% of the inspected dams, with embankment dams showing a slightly higher ratio. Damage to embankment dams included relatively wide and/or long cracks mainly on the crest of earthfill dams, cracks in the impervious membrane of asphalt faced rockfill dams (AFRDs) and temporary increase in leakage. However, none of the dams under the jurisdiction of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) sustained damage severe enough to threaten the safety of the dam, although an old earthfill embankment dam for an irrigation pond located outside the area regulated by the River Law was breached due to the earthquake. In this paper, the results of special safety inspections are summarized, as well as those of subsequent detailed investigations of several embankment dams under the jurisdiction of MILT. The features of the earthquake motions observed at the dam foundations during the earthquake and their effects on the behavior of embankment dams are also discussed. [Copyright &y& Elsevier]